Looks like nutrient deficiency. Probably iron. Do you have a nutrient-rich substrate? Swords are root-feeders i.e. take a lot of their nutrients from their roots.
Could also be lack of light (but doubt it) - how much do you have?
I use the Dupla tablets as shown above and can recomend them. The roots wil soon find them. It's best to breack them into three and then put them far enough under that any bottom dwellers like corys can't dig them up.